Gallium nitride crystal is expected as a material for short wavelength laser light emission of about 400 nm in wavelength or a material for high temperature and high speed transistor, but homogeneous monocrystalline gallium nitride crystal has not been obtained yet at the present.
A conventional method for manufacturing semiconductor device is described by referring to FIG. 6(a), FIG. 6(b), and FIG. 6(c). As shown in FIG. 6(a) and FIG. 6(b), a zinc oxide layer 2 of film thickness of 500 Angstroms is formed on a monocrystalline substrate 1 made of sapphire at temperature of 500.degree. C. in an oxygen atmosphere by using a sputtering apparatus. Next, the monocrystalline substrate 1 forming the zinc oxide layer 2 is dismounted from the sputtering apparatus, the temperature of the monocrystalline substrate 1 is raised to 1000.degree. C. by using a halide VPE apparatus, and ammonia and gallium chloride produced by reaction between metal gallium and hydrochloric acid gas are supplied into the halide VPE apparatus, thereby forming a gallium nitride crystal 3 in a film thickness of about 100 .mu.m as shown in FIG. 6(c).
In this way, the crystal growth of gallium nitride crystal is generally conducted by heteroepitaxial growth using sapphire as monocrystalline substrate 1.
Incidentally, the lattice constant of the zinc oxide layer 2 is 3.250 Angstroms in the a-axis direction of hexagonal system, which is close to the lattice constant 3.189 Angstroms of gallium nitride, and it is hence used to alleviate lattice mismatching between sapphire and gallium nitride.
However, when forming gallium nitride crystal by the conventional method for manufacturing semiconductor device, since the monocrystalline substrate is heated to high temperature of about 1000.degree. C. which is an appropriate temperature for growth of gallium nitride, oxygen may be vaporized from the zinc oxide layer so that the zinc oxide layer decompose, or the zinc oxide layer may be etched by ammonium chloride formed by reaction of raw material gas. As a result, the zinc oxide layer may be non-uniform, and it was therefore hard to form homogeneous gallium nitride crystal over the entire area on the zinc oxide layer at high reproducibility.